Puberty snooze: teenagers sink into the couch

Get a load of this ... More than 20 per cent of teenagers do no excercise at all.

Teenagers are less likely to smoke and drink alcohol but are far more physically inactive than a decade ago, new statistics show.

A report on health risks released today by the Australian Institute of Health and Welfare found that smoking was down in all age groups.

Teenagers were the only age group less likely to have consumed alcohol, with the proportion of secondary school students aged 12 to 17 drinking alcohol at risky levels declining between 1984 and 2008.

But physical inactivity was a particular problem for teenagers - for whom an hour or more each day of at least moderate intensity exercise is recommended, compared to 30 minutes for adults.

The proportion of teenagers aged 15 to 17 who did no exercise increased from 16 to 22 per cent between 1995 and 2008, including more females than males.

A total of 85 per cent of teenagers did less than the recommended hour of exercise each day, and those who did exercise spent 1.3 hours less time doing so each fortnight.

The proportion of adults who were overweight or obese increased from 56 to 61 per cent and was worse among men - with 68 per cent classified as overweight or obese, compared to 55 per cent of women.

For teenagers, 28 per cent were overweight or obese in 2008, compared to 23 per cent in 1995.

Professor David Dunstan, from the Baker IDI Heart and Diabetes Institute, said physical inactivity was likely to be underestimated in the data, because it was based on people's own reports of their activity levels.

"We know that when you put devices on people to track their movements it's a substantial underestimate of the time they've spent sedentary," he said.

Professor Dunstan said there were likely to be multiple reasons for declines in physical activity, particularly among teenagers, but "we have to factor in the electronic age".

The report's authors also noted that modern lifestyles were conducive to lower levels of physical activity.

"Many people use cars for transport and increasingly people are working full-time and in jobs that do not involve physical activity," they said.